Filter material may be placed over photodetector devices formed in an integrated circuit. This filter material may be transparent to selected light frequencies or ranges of frequencies so that only light corresponding to a selected color reaches an underlying device. More than one filter material type may be used so that selected device groups each receive light corresponding to a distinct color. For example, three materials, each transmitting a unique one of red, green, and blue light, may be placed in a pattern over underlying devices.
Light transmitting material is typically formed over photodetector devices using conventional photolithographic methods. The light transmitting material is commonly a thick film of colored resist. Briefly, a material transmitting light for a color to be sensed is spun on to a processed wafer. The material is then exposed, developed, and etched using standard photolithographic methods so that it is patterned as individual islands over corresponding photo detecting devices. As a result, for a large color sensing device array, many individual color filter islands overlie photodetector devices in the integrated circuit. This process may be repeated using different material or materials to form additional individual filters transmitting light of different colors. The molecular structures of the thick films of resist typically produce poor imaging because the resist is usually optimized to transmit the desired color, not for etch resolution. Thick film molecular lengths often exceed the sub-micrometer resolution required in contemporary integrated circuits (ICs) so that precise filter placement is difficult. A more significant problem is that one or more of the resist material islands may loosen or fall off (on the order of a few islands per million).
What is required, therefore, is a method for forming color filters on ICs that allows deep sub-micrometer alignment and etch resolution while allowing the use of materials optimized to transmit colored light. In addition, the structure should offer improved adhesion between the filter and the IC.